I had a tapping sound in my '94 S10 Blazer for about 4 years. I and a mechanic looked for it to no avail, checking plug wires, using a stethescope, and so on. The mechanic thought it was a noisy injector. Ran great all that time. Finally decided to change plugs and such recently. The first plug wire I started to pull to change the plug was not on the plug solidly and the spark had been jumping the gap to the plug all that time. The moral of the story is to check plug wires both by pushing (which we did) and pulling (which we didn't). No more "tapping" sound.
